Al-Bearing Scorodite (Scorodite—Mansfieldite Series) from Hemerdon Ball Mine, Plympton, Tavistock District, Devon, United Kingdom: Single-Crystal X-Ray Diffraction, Chemistry and Vibrational Spectroscopy

The Al-bearing scorodite from the Hemerdon Ball Mine (HBM) was studied using electron microscopy and microprobe analysis, single-crystal X-ray diffraction, infrared, and Raman spectroscopy. The crystal chemistry formula of Al-bearing scorodite is expressed as Fe3+0.87Al3+0.16(As0.97O4)·H2O. The calculated d-spacings and unit-cell parameters of Al-bearing scorodite are slightly affected by the substitution of Al for Fe in the octahedral sites. The Al-bearing scorodite HBM crystalizes in the Pbca space group with the following unit-cell lattice parameters: a = 8.92882(14) Å; b = 10.02217(14) Å; c = 10.30525(15) Å; V(Å) = 922.18(2) and Z = 8. The lattice structure becomes slightly distorted by the formation of the Fe,Al-OH octahedron, which leads to a compression of the newly formed octahedron along the a* ^ b* direction and an expansion of the Fe-OH octahedron along the c* direction. The incorporation of Al3+ has a strong effect on the tilting angle of the Fe,Al-OH octahedron in the b* ^ c* crystallographic direction. The refined structure suggests that Al3+ occupies the octahedral sites alongside Fe3+, leading to a distortion of the Fe,Al-OH octahedron. Infrared and Raman spectroscopy exhibit a doublet at 820 and 800 cm−1, and at 810 and 800 cm−1 ascribed to the Fe,Al-O-OAsO3 group. The 799–800 cm−1 Raman region is assigned to the Fe–O–As group (at 798 and 803 cm−1), whereas the 810–814 cm−1 region is ascribed to a band resulting from the AsO43− [υ1 (A1) symmetric stretching vibrational modes], indicative of the Fe,Al–OH–As group in both Al-bearing scorodite and mansfieldite.